- Email: [email protected]
Voltammetric and ellipsometric study of electrically conducting films of poly(o-toluidine) in different acid media
PERGAMON
Electrochimica Acta 44 (1999) 1973±1979
Voltammetric and ellipsometric study of electrically conducting ®lms of poly(o-toluidine) in dierent acid media J.O. Zerbino a, *, M.I. Florit a, A. Maltz b a
INIFTA (UNLP, CONICET, CIC), C.C. 16 Suc. 4 (1900) La Plata, Argentina Dep. de Matematica, Fac. C. Exactas, UNLP Calle 115 y 500 (1900) La Plata, Argentina
b
Received 27 July 1998
Abstract Electrochemically synthetized poly(o-toluidine) (POT) ®lms were studied in 3.7 M HClO4 and 1 M HClO4 solutions. The optical indices n ÿ i k, at 0.1 V and 0.6 V were calculated in the 405 nm < l <580 nm range. Undoped ®lms show an increase in thickness, d, of about 10% related to the doped state. The decrease of the electrolyte concentration yield a decrease in ``d'' and an increase in ``n'', as compared to a more compact structure of the ®lm. The pseudacapacitive charge, Q, corresponding to the reduction of the doped state, was measured as a function of the thickness. # 1999 Elsevier Science Ltd. All rights reserved.
1. Introduction Electroactive polymer ®lms such as those derived from pyzrrole, aniline, thiophene and o-aminophenol are receiving growing attention due to their applications in microelectronic devices, analytical sensors and displays. Poly(o-toluidine) (POT) is a polymer very similar to poly(aniline) (PANI). Particularly, the switching behaviour insulator/conductor of POT ®lms, electrochemically synthesized, has been quite extensively studied [1± 3]. Moreover, POT ®lms have been characterized through voltammetry, EIS and IR studies [4, 5]. On the other hand, ellipsometric in-situ studies have been performed on electrodes modi®ed with dierent polymer ®lms, in particular polythiophene (PTP), with the purpose of ascertaining the use of this technique for thickness and optical indices measurements [6±9]. Here we present a report on the voltammetric and ellipsometric behaviour of POT in the reduced and the half-oxidized state as a function of the electrode potential. The correlation among thickness, optical constants, voltammetric charge, potential and pH,
* Corresponding author. E-mail: [email protected]
parameters related with the relative concentration of oxidized and reduced sites in the polymer matrix, improves the knowledge of the interface, yielding a microscopic in-situ picture of the restructuration processes, that take place, as a function of the potential, in the conducting ®lm.
2. Experimental The cells for the electrochemical and optical measurements have been described elsewhere [4, 6]. Potentials in the text are referred to the Reversible Hydrogen Electrode (RHE). The measurements were made at 405, 450, 492, 546 and 580 nm. The refraction indices of the substrate were obtained at 0.1 V, in acid aqueous solutions of HClO4 3.7 M, from the ellipsometric parameters of the recently polished Au electrode. The resulting values are in good agreement with previously reported data [10]. Monomer was puri®ed in a Perkin±Elmer 251 Autoannular Still distillation column. It was added in the optical cell containing HClO4 3.7 M solutions resulting a concentration 0.5 M in o-toluidine. The POT ®lms were electrochemically grown in this sol-
0013-4686/99/$ - see front matter # 1999 Elsevier Science Ltd. All rights reserved. PII: S 0 0 1 3 - 4 6 8 6 ( 9 8 ) 0 0 3 0 6 - 5
1974
J.O. Zerbino et al. / Electrochimica Acta 44 (1999) 1973±1979
ution, by cycling the potential between 0.1 and 1.1 V, at room temperature. The changes in the ellipsometric parameters, D and C were measured at 0.1 and 0.6 V, following two dierent experimental procedures.
2.1. Procedure I The parameters D and c were measured at both potentials 0.1 and 0.6 V after cycling for 5 min time periods.
2.2. Procedure II
Fig. 1. Voltammetric scan for the growing ®lm of poly-(otoluidine) (POT), in 3.7 M HClO4 + 0.5 M o-toluidine (OT), after cycling potential 20 min. Dotted line shows the pseudocapacitive current involves in the transformation between the insulator and the conducting states, E = 0.1 and E = 0.6 V, respectively. Q = 1.20 mCoul cm ÿ 2. Potential values are referred to the RHE.
After performing procedure I, the circuit was opened at 0.1 V and the electrolyte in the cell was replaced by HClO4 1 M solution. Then, D and C were measured in the new electrolyte solution ®rst at 0.1 V and then at 0.6 V, after cycling during dierent times periods in this potential range. It should be remarked that special attention was taken to avoid the modi®cation of the optical alignment of the electrode, during the change of electrolyte.
Fig. 2. Ellipsommetric parameters measured at the conducting state, E = 0.6 V, after dierent cycling times in HClO4 3.7 M + 0.5 M o-toluidine. (w) and (q) experimental points. The full circles correspond to a ®lm of optical constants: n = 1.3647 and k = 0.05834. The ®gures show the thickness in nm. (r) results in HClO4 1 M.
J.O. Zerbino et al. / Electrochimica Acta 44 (1999) 1973±1979
3. Results and discussion The voltammetric behaviour of POT is very similar to that reported for PANI and other arylamine derived polymers [11, 12]. During the positive scan the voltammogram shows two couples of redox current peaks (Fig. 1). On inversion of the sweep direction at 0.6 V, an appreciable capacitive current is noticed. The reduction peak of the ®rst redox process is displaced in the negative direction from the anodic peak potential. This shows that the redox couple should correspond to a quasi-reversible process and the activity factors for the oxidized and reduced species should be quite dierent. According to Procedure I the changes in the ellipsometric parameter, D and C, measured at 0.6 V and l = 546 nm, corresponding to the doped ®lm are shown in Fig. 2. The points lie on a S-shaped curve. The D and C values measured at 0.1 V, show a periodical change resulting three overlying curves (Fig. 3). The experimental data were ®tted using the gradient techniques [10]. The refraction index, n, the absorption coecient, k, and thickness, d, were calculated. The model used for this calculation assumes a single homogeneous ®lm of dierent thickness. The functions to be
1975
minimized were either 0 2 2 2 the ex the G S@Dex ÿ D C ÿ C Qj ÿ adhj ij ij ij ij or
0
the G S@Dex ij ÿ Dij
2
the Cex ij ÿ Cij
2
I
II
where the subindex i corresponds to the dierent optical data measured at li and the subindex j to the dierent measured charges Q. This global optimization method allows the calculation in the case of Eq. (II) of the indices ni ÿ ki and the thicknesses dj, since D theij and C theij are function of these parameters. The higher number of data relate to the number of adjusted parameters allows us to ®nd out univoque solutions. In the case of Eq. (I) are introduced additional Qi data obtaining the parameters a and b. The ®tted values of n, k and d obtained using Eq. (I) were used as initial values in the condition of Eq. (II). If the new n, k, and d results similar to those previously calculated, this proves that the third term in Eq. (I) leads towards the correct solutions.
Fig. 3. As Fig. 2 but measured at the insulator state, E = 0.1 V. The full circles correspond to a ®lm of optical constants: n = 1.5187 and k = 0.000, for a thickness ®lm increase of 100 AÊ.
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J.O. Zerbino et al. / Electrochimica Acta 44 (1999) 1973±1979
Fig. 4. Correlation between the voltammetric charge and the POT ®lm thickness (d). (q) for the reduced and (r) for the oxidized state. For the thicker ®lms, an increase of about 10% in thickness is observed between the doped and undoped POT ®lms.
If 5 li and 6 Qj are selected the optical data are 30 values Dij and 30 values Cij. Eq. (I) uses 66 data and the number of ®tted parameters, ``p'', are 18: that is 5 ni, 5 ki, 6 dj, a and b. Otherwise, in the condition of Eq. (II) the data are 60 and the adjustable parameters 16. As the number of experimental data exceeds the number of parameters to be adjusted the optimization converges after ``m'' iteration, to theoretical values Dij, Cij. The convergence is ful®lled for m increasing when
Fig. 5. Calculated values of ``n'' of the reduced ®lm as function of wavelength. (w) in the range of d: 80 nm < d < 450 nm. (q) 290 nm < d < 520 nm (.) d = 511 nm.
(a) the euclidean norm of the arrangement, pm ÿ pm + 1, tends to 0, (b) G( pm) > G( pm + 1) > G( pm + 2) and (c) @Gm/@p tends to 0. The optical indices calculated are average values, which ®t the optical data in the range of selected thickness. Afterwards, using these indices and thicknesses as initial values in a new optimization step, ®tting the data corresponding to smaller sets of d (a series of either two or three sequential d), the variation of the optical indices on d can be calculated [6±10]. The theoretical curve obtained using Eq. (II) was plotted in the Figs. 2 and 3 for increases of d = 10 nm. The theoretical predicted values of D and C were plotted as larger full points on the pointed curve. Thicknesses of about d = 675 nm were attained, measuring at 0.6 V, after a total time of 100 min cycling. Similar experiment yields d = 374 nm after 80 min of cycling (Fig. 2). During the ®rst 45 min of cycling was observing a very small change in the optical parameters. The cathodic charge, Q, involved in the scans at 100 mV/s from 0.6 to 0.1 V were measured after each cycling time periods and the corresponding d values were calculated using Eq. (I). The d-values are plotted
Fig. 6. Calculated values of ``n'' of the doped ®lm as function of wavelength. (w) in the range of d: 75 nm < d < 380 nm. (q) 250 nm < d < 410 nm (.) d = 440 nm.
J.O. Zerbino et al. / Electrochimica Acta 44 (1999) 1973±1979
1977
Fig. 8. Calculated values of ``n'' of the reduced ®lm as function of wavelength. (.) d = 511 nm, HClO4 3.7 M + 0.5 M otoluidine. Empty marks corresponds to HClO4 1 M solutions: (q) after 1 cycle, (r) 2 cycles, (t) 10 cycles, (r) 34 cycles. d about 460 nm.
against Q in Fig. 4. The points ®t curves Q = a * d b. The resulting values of 0.1 and 0.6 V are a = 0.1903, b = 0.6934 and a = 0.2964, b = 0.6368 respectively; d (nm), Q (mCoul.cm ÿ 2). The exponent b lower than 1 points to a decrease in the eciency of the charge±decharge process for increasing thickness. This may be caused either by a decrease in the kinetics of the reduction process or a
Fig. 7. Optical thickness ``d'' of the doped and undoped ®lm against electrical charge Q determined from integration of the cathodic current for the reduction of the ®lm (Ã), (q) and (.) as in Figs. 5 and 6.
decrease in the compactness of the deposits. Probably both eects contribute to this downfall in the doping/ undoping processes. The stepwise increase of d, assuming n and k to remain constants over at least two cycling steps, is one criterion for the unequivocal solution of the ellipsometric equations [6±10]. Fig. 5 shows the n ®tted
Fig. 9. Calculated values of ``n'' of the doped ®lm as function of wavelength. (.) d = 436 nm, HClO4 3.7 M + 0.5 M o-toluidine. Empty marks corresponds to HClO4 1 M solutions: (q) after 1 cycle, (r) 2 cycles, (t) 34 cycles. d about 390 nm.
Fig. 10. Calculated values of d for the undoped ®lm (w) and the doped ®lm (q) as function of the number of cycles in HClO4 1 M solutions. Full marks reference values of d in HClO4 3.7 M + 0.5 M o-toluidine.
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J.O. Zerbino et al. / Electrochimica Acta 44 (1999) 1973±1979
Fig. 11. Voltammetric scan of Au/POT electrode in 1 M HClO4, at n = 100 mV s ÿ 1, Q = 79.91 mCoul cm ÿ 2. Dotted line shows the charge ¯ux between the insulator and the conducting states, E = 0.1 V and E = 0.6 V, respectively.
values corresponding to the reduced state of POT. The k values result equal to 0. The refraction indices corresponding to six thicknesses in the range 80 < d < 450 nm result higher that those calculated in the 290 < d < 520 range. This shows a decrease in the compactness of the ®lm or an increase in roughness for the thicker thicknesses. The n indices in the 290 < d < 520 range are very similar to those calculated for one thickness, d = 511 nm. The dierence for l = 405 nm is probably related to eects of non-homogeneity in the structure of the ®lm which increase the errors at the shorter wavelengths, (lr d) [13]. The ®tted programme routine considering an auxiliary index n ÿ i(k 0 )2 allows only variations of k in the nonnegative domain. At the l values where k tends to 0 the border condition k = 0 restrains the variation of k. In the systems including at least one k tending to 0 the experimental data presents good convergence in the optimization ®tting using only one thickness (Figs 6±9). Fig. 6 shows the ®tted values of n and k for the doped state. Similarly to that observed in Fig. 5 as the thickness increase, n decrease, indicating less compact ®lms. The results at l < 450 nm and l > 580 were uncertain and were not included in this ®tting. This is probably due to the increasing eect of inhomogeneity at the shortest l (l r d) and the high absorption and low intensity of the lamp at the longer l. The ®tted values of d (Fig. 7) ®tted in the range: 450 nm < l < 580 nm shows good convergence and agree with that obtained for the undoped state. The Procedure II was carried out in the following way after 80 min of cycling the electrolyte was
replaced. The pointed line indicates the corresponding variations of D and C obtained in HClO4 3.7 M and HClO4 1 M. At 0.1 V and l = 546 nm (Fig. 3) the ellipsometric changes occurs at about D = 1008 and C = 448. The eect of increase the pH of the electrolyte is shown in the Figs. 8±11. The ®lm decreases in thickness and increases in compactness [13], probably displacing solvent and anions. The eect is observed in the ®rst cycles attaining quickly a new stable condition. The current of the cathodic peak decreases about 30% related to that in HClO4 3.7 M. This peak is broader and the charge decreases about 12%. For the polyanilines and other derivate polymers, compared with other conducting polymers, relatively scarce information has been reported about their fundamental physical properties in solution. At least four possible forms have been discussed [11, 12]. Magnetic susceptibility studies are in agreement with the proposal that protonation leads to phase segregation of unand full-protonated domains [12]. The correlation of the information given by the dierent techniques will provide a detailed description of the dierent processes taking place in these complex systems.
Acknowledgements The authors are very grateful to Dr J. Caram for the puri®cation of the OT. This work was supported by the ``Consejo Nacional de Investigaciones Cienti®cas'' (CONICET) and the ``Comision de Investigaciones Cienti®cas de la Provincia de Buenos Aires'' (CIC). M.I.F. are members of the Research Career of CONICET and J.O.Z. of the Research Career of CIC.
References [1] E.M. Genies, J.F. Penneau, M. Lapkowsi, New J. Chem. 12 (1988) 181. [2] Y. Wen, W.F. Focke, G.E. Wnekk, A. Ray, A.G. MacDiarmid, J. Phys. Chem. 93 (1989) 495. [3] I. Fujita, M. Ischiguchi, H. Shiota, T. Danjo, K. Josai, J. Appl. Polym. Sci. 44 (1992) 987. [4] M.I. Florit, J. of Electroanal. Chem. 408 (1996) 257. [5] E.M. Andrade, F.V. Molina, M.I. Florit, D. Posadas, J. of Electroanal. Chem. () (in) press . [6] C. Barbero, J. Zerbino, L. Sereno, D. Posadas, Electroanal. Chem. 32 (4) (1987) 693. [7] W.J. Plieth, J. Zerbino, C. Lahmann, G. Kossmehl, J. Electroanal. Chem. 274 (1989) 213. [8] J. Zerbino, W.J. Plieth, G. Kossmehl, J. Electroanal. Chem. 260 (1989) 361. [9] J. Zerbino, W.J. Plieth, G. Kossmehl, J. Appl. Electrochem. 21 (1991) 935.
J.O. Zerbino et al. / Electrochimica Acta 44 (1999) 1973±1979 [10] M.E. Vela, J.O. Zerbino, A.J. Arvia, Thin Solid Films 233 (1993) 82. [11] Salaneck, W.R., Lundstrom, I., Liedberg, B., Hasan, M.A., Erlandsson, R., Konradsson, P., MacDiarmid, A.G., Somarisi, N.L.D. in Electronic Properties of Polymers and Related Compounds. Eds. Kuzmany, H., Mehring, U., Roth, S. 1985, p. 218.
1979
[12] Bredas, J.L., in Electronic Properties of Conjugated Polymers. Proceeding of International Winter School, Kirchberg. Eds. Springer, Berlin, 1987. [13] Aspnes, D.E. Handbook of Optical Constants of Solids. Ed. Palik ED. Academic Press, New York. Chap. 5. 1985, p. 105.
Electrochimica Acta 44 (1999) 1973±1979
Voltammetric and ellipsometric study of electrically conducting ®lms of poly(o-toluidine) in dierent acid media J.O. Zerbino a, *, M.I. Florit a, A. Maltz b a
INIFTA (UNLP, CONICET, CIC), C.C. 16 Suc. 4 (1900) La Plata, Argentina Dep. de Matematica, Fac. C. Exactas, UNLP Calle 115 y 500 (1900) La Plata, Argentina
b
Received 27 July 1998
Abstract Electrochemically synthetized poly(o-toluidine) (POT) ®lms were studied in 3.7 M HClO4 and 1 M HClO4 solutions. The optical indices n ÿ i k, at 0.1 V and 0.6 V were calculated in the 405 nm < l <580 nm range. Undoped ®lms show an increase in thickness, d, of about 10% related to the doped state. The decrease of the electrolyte concentration yield a decrease in ``d'' and an increase in ``n'', as compared to a more compact structure of the ®lm. The pseudacapacitive charge, Q, corresponding to the reduction of the doped state, was measured as a function of the thickness. # 1999 Elsevier Science Ltd. All rights reserved.
1. Introduction Electroactive polymer ®lms such as those derived from pyzrrole, aniline, thiophene and o-aminophenol are receiving growing attention due to their applications in microelectronic devices, analytical sensors and displays. Poly(o-toluidine) (POT) is a polymer very similar to poly(aniline) (PANI). Particularly, the switching behaviour insulator/conductor of POT ®lms, electrochemically synthesized, has been quite extensively studied [1± 3]. Moreover, POT ®lms have been characterized through voltammetry, EIS and IR studies [4, 5]. On the other hand, ellipsometric in-situ studies have been performed on electrodes modi®ed with dierent polymer ®lms, in particular polythiophene (PTP), with the purpose of ascertaining the use of this technique for thickness and optical indices measurements [6±9]. Here we present a report on the voltammetric and ellipsometric behaviour of POT in the reduced and the half-oxidized state as a function of the electrode potential. The correlation among thickness, optical constants, voltammetric charge, potential and pH,
* Corresponding author. E-mail: [email protected]
parameters related with the relative concentration of oxidized and reduced sites in the polymer matrix, improves the knowledge of the interface, yielding a microscopic in-situ picture of the restructuration processes, that take place, as a function of the potential, in the conducting ®lm.
2. Experimental The cells for the electrochemical and optical measurements have been described elsewhere [4, 6]. Potentials in the text are referred to the Reversible Hydrogen Electrode (RHE). The measurements were made at 405, 450, 492, 546 and 580 nm. The refraction indices of the substrate were obtained at 0.1 V, in acid aqueous solutions of HClO4 3.7 M, from the ellipsometric parameters of the recently polished Au electrode. The resulting values are in good agreement with previously reported data [10]. Monomer was puri®ed in a Perkin±Elmer 251 Autoannular Still distillation column. It was added in the optical cell containing HClO4 3.7 M solutions resulting a concentration 0.5 M in o-toluidine. The POT ®lms were electrochemically grown in this sol-
0013-4686/99/$ - see front matter # 1999 Elsevier Science Ltd. All rights reserved. PII: S 0 0 1 3 - 4 6 8 6 ( 9 8 ) 0 0 3 0 6 - 5
1974
J.O. Zerbino et al. / Electrochimica Acta 44 (1999) 1973±1979
ution, by cycling the potential between 0.1 and 1.1 V, at room temperature. The changes in the ellipsometric parameters, D and C were measured at 0.1 and 0.6 V, following two dierent experimental procedures.
2.1. Procedure I The parameters D and c were measured at both potentials 0.1 and 0.6 V after cycling for 5 min time periods.
2.2. Procedure II
Fig. 1. Voltammetric scan for the growing ®lm of poly-(otoluidine) (POT), in 3.7 M HClO4 + 0.5 M o-toluidine (OT), after cycling potential 20 min. Dotted line shows the pseudocapacitive current involves in the transformation between the insulator and the conducting states, E = 0.1 and E = 0.6 V, respectively. Q = 1.20 mCoul cm ÿ 2. Potential values are referred to the RHE.
After performing procedure I, the circuit was opened at 0.1 V and the electrolyte in the cell was replaced by HClO4 1 M solution. Then, D and C were measured in the new electrolyte solution ®rst at 0.1 V and then at 0.6 V, after cycling during dierent times periods in this potential range. It should be remarked that special attention was taken to avoid the modi®cation of the optical alignment of the electrode, during the change of electrolyte.
Fig. 2. Ellipsommetric parameters measured at the conducting state, E = 0.6 V, after dierent cycling times in HClO4 3.7 M + 0.5 M o-toluidine. (w) and (q) experimental points. The full circles correspond to a ®lm of optical constants: n = 1.3647 and k = 0.05834. The ®gures show the thickness in nm. (r) results in HClO4 1 M.
J.O. Zerbino et al. / Electrochimica Acta 44 (1999) 1973±1979
3. Results and discussion The voltammetric behaviour of POT is very similar to that reported for PANI and other arylamine derived polymers [11, 12]. During the positive scan the voltammogram shows two couples of redox current peaks (Fig. 1). On inversion of the sweep direction at 0.6 V, an appreciable capacitive current is noticed. The reduction peak of the ®rst redox process is displaced in the negative direction from the anodic peak potential. This shows that the redox couple should correspond to a quasi-reversible process and the activity factors for the oxidized and reduced species should be quite dierent. According to Procedure I the changes in the ellipsometric parameter, D and C, measured at 0.6 V and l = 546 nm, corresponding to the doped ®lm are shown in Fig. 2. The points lie on a S-shaped curve. The D and C values measured at 0.1 V, show a periodical change resulting three overlying curves (Fig. 3). The experimental data were ®tted using the gradient techniques [10]. The refraction index, n, the absorption coecient, k, and thickness, d, were calculated. The model used for this calculation assumes a single homogeneous ®lm of dierent thickness. The functions to be
1975
minimized were either 0 2 2 2 the ex the G S@Dex ÿ D C ÿ C Qj ÿ adhj ij ij ij ij or
0
the G S@Dex ij ÿ Dij
2
the Cex ij ÿ Cij
2
I
II
where the subindex i corresponds to the dierent optical data measured at li and the subindex j to the dierent measured charges Q. This global optimization method allows the calculation in the case of Eq. (II) of the indices ni ÿ ki and the thicknesses dj, since D theij and C theij are function of these parameters. The higher number of data relate to the number of adjusted parameters allows us to ®nd out univoque solutions. In the case of Eq. (I) are introduced additional Qi data obtaining the parameters a and b. The ®tted values of n, k and d obtained using Eq. (I) were used as initial values in the condition of Eq. (II). If the new n, k, and d results similar to those previously calculated, this proves that the third term in Eq. (I) leads towards the correct solutions.
Fig. 3. As Fig. 2 but measured at the insulator state, E = 0.1 V. The full circles correspond to a ®lm of optical constants: n = 1.5187 and k = 0.000, for a thickness ®lm increase of 100 AÊ.
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J.O. Zerbino et al. / Electrochimica Acta 44 (1999) 1973±1979
Fig. 4. Correlation between the voltammetric charge and the POT ®lm thickness (d). (q) for the reduced and (r) for the oxidized state. For the thicker ®lms, an increase of about 10% in thickness is observed between the doped and undoped POT ®lms.
If 5 li and 6 Qj are selected the optical data are 30 values Dij and 30 values Cij. Eq. (I) uses 66 data and the number of ®tted parameters, ``p'', are 18: that is 5 ni, 5 ki, 6 dj, a and b. Otherwise, in the condition of Eq. (II) the data are 60 and the adjustable parameters 16. As the number of experimental data exceeds the number of parameters to be adjusted the optimization converges after ``m'' iteration, to theoretical values Dij, Cij. The convergence is ful®lled for m increasing when
Fig. 5. Calculated values of ``n'' of the reduced ®lm as function of wavelength. (w) in the range of d: 80 nm < d < 450 nm. (q) 290 nm < d < 520 nm (.) d = 511 nm.
(a) the euclidean norm of the arrangement, pm ÿ pm + 1, tends to 0, (b) G( pm) > G( pm + 1) > G( pm + 2) and (c) @Gm/@p tends to 0. The optical indices calculated are average values, which ®t the optical data in the range of selected thickness. Afterwards, using these indices and thicknesses as initial values in a new optimization step, ®tting the data corresponding to smaller sets of d (a series of either two or three sequential d), the variation of the optical indices on d can be calculated [6±10]. The theoretical curve obtained using Eq. (II) was plotted in the Figs. 2 and 3 for increases of d = 10 nm. The theoretical predicted values of D and C were plotted as larger full points on the pointed curve. Thicknesses of about d = 675 nm were attained, measuring at 0.6 V, after a total time of 100 min cycling. Similar experiment yields d = 374 nm after 80 min of cycling (Fig. 2). During the ®rst 45 min of cycling was observing a very small change in the optical parameters. The cathodic charge, Q, involved in the scans at 100 mV/s from 0.6 to 0.1 V were measured after each cycling time periods and the corresponding d values were calculated using Eq. (I). The d-values are plotted
Fig. 6. Calculated values of ``n'' of the doped ®lm as function of wavelength. (w) in the range of d: 75 nm < d < 380 nm. (q) 250 nm < d < 410 nm (.) d = 440 nm.
J.O. Zerbino et al. / Electrochimica Acta 44 (1999) 1973±1979
1977
Fig. 8. Calculated values of ``n'' of the reduced ®lm as function of wavelength. (.) d = 511 nm, HClO4 3.7 M + 0.5 M otoluidine. Empty marks corresponds to HClO4 1 M solutions: (q) after 1 cycle, (r) 2 cycles, (t) 10 cycles, (r) 34 cycles. d about 460 nm.
against Q in Fig. 4. The points ®t curves Q = a * d b. The resulting values of 0.1 and 0.6 V are a = 0.1903, b = 0.6934 and a = 0.2964, b = 0.6368 respectively; d (nm), Q (mCoul.cm ÿ 2). The exponent b lower than 1 points to a decrease in the eciency of the charge±decharge process for increasing thickness. This may be caused either by a decrease in the kinetics of the reduction process or a
Fig. 7. Optical thickness ``d'' of the doped and undoped ®lm against electrical charge Q determined from integration of the cathodic current for the reduction of the ®lm (Ã), (q) and (.) as in Figs. 5 and 6.
decrease in the compactness of the deposits. Probably both eects contribute to this downfall in the doping/ undoping processes. The stepwise increase of d, assuming n and k to remain constants over at least two cycling steps, is one criterion for the unequivocal solution of the ellipsometric equations [6±10]. Fig. 5 shows the n ®tted
Fig. 9. Calculated values of ``n'' of the doped ®lm as function of wavelength. (.) d = 436 nm, HClO4 3.7 M + 0.5 M o-toluidine. Empty marks corresponds to HClO4 1 M solutions: (q) after 1 cycle, (r) 2 cycles, (t) 34 cycles. d about 390 nm.
Fig. 10. Calculated values of d for the undoped ®lm (w) and the doped ®lm (q) as function of the number of cycles in HClO4 1 M solutions. Full marks reference values of d in HClO4 3.7 M + 0.5 M o-toluidine.
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J.O. Zerbino et al. / Electrochimica Acta 44 (1999) 1973±1979
Fig. 11. Voltammetric scan of Au/POT electrode in 1 M HClO4, at n = 100 mV s ÿ 1, Q = 79.91 mCoul cm ÿ 2. Dotted line shows the charge ¯ux between the insulator and the conducting states, E = 0.1 V and E = 0.6 V, respectively.
values corresponding to the reduced state of POT. The k values result equal to 0. The refraction indices corresponding to six thicknesses in the range 80 < d < 450 nm result higher that those calculated in the 290 < d < 520 range. This shows a decrease in the compactness of the ®lm or an increase in roughness for the thicker thicknesses. The n indices in the 290 < d < 520 range are very similar to those calculated for one thickness, d = 511 nm. The dierence for l = 405 nm is probably related to eects of non-homogeneity in the structure of the ®lm which increase the errors at the shorter wavelengths, (lr d) [13]. The ®tted programme routine considering an auxiliary index n ÿ i(k 0 )2 allows only variations of k in the nonnegative domain. At the l values where k tends to 0 the border condition k = 0 restrains the variation of k. In the systems including at least one k tending to 0 the experimental data presents good convergence in the optimization ®tting using only one thickness (Figs 6±9). Fig. 6 shows the ®tted values of n and k for the doped state. Similarly to that observed in Fig. 5 as the thickness increase, n decrease, indicating less compact ®lms. The results at l < 450 nm and l > 580 were uncertain and were not included in this ®tting. This is probably due to the increasing eect of inhomogeneity at the shortest l (l r d) and the high absorption and low intensity of the lamp at the longer l. The ®tted values of d (Fig. 7) ®tted in the range: 450 nm < l < 580 nm shows good convergence and agree with that obtained for the undoped state. The Procedure II was carried out in the following way after 80 min of cycling the electrolyte was
replaced. The pointed line indicates the corresponding variations of D and C obtained in HClO4 3.7 M and HClO4 1 M. At 0.1 V and l = 546 nm (Fig. 3) the ellipsometric changes occurs at about D = 1008 and C = 448. The eect of increase the pH of the electrolyte is shown in the Figs. 8±11. The ®lm decreases in thickness and increases in compactness [13], probably displacing solvent and anions. The eect is observed in the ®rst cycles attaining quickly a new stable condition. The current of the cathodic peak decreases about 30% related to that in HClO4 3.7 M. This peak is broader and the charge decreases about 12%. For the polyanilines and other derivate polymers, compared with other conducting polymers, relatively scarce information has been reported about their fundamental physical properties in solution. At least four possible forms have been discussed [11, 12]. Magnetic susceptibility studies are in agreement with the proposal that protonation leads to phase segregation of unand full-protonated domains [12]. The correlation of the information given by the dierent techniques will provide a detailed description of the dierent processes taking place in these complex systems.
Acknowledgements The authors are very grateful to Dr J. Caram for the puri®cation of the OT. This work was supported by the ``Consejo Nacional de Investigaciones Cienti®cas'' (CONICET) and the ``Comision de Investigaciones Cienti®cas de la Provincia de Buenos Aires'' (CIC). M.I.F. are members of the Research Career of CONICET and J.O.Z. of the Research Career of CIC.
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